While having different project pages for different versions of a game is irritating in some cases, this quality guideline does not apply imo.  And if you double read it, it actually states different versions of a project. 

I often see demo and full version as different projects. Given the rudimentary mechanics provided by itch, this is almost necessary in some cases.

While you can set your price to pay what you want     and make the stable full release  threshold x amount, you might not want to give out the testing version to everyone. Or more importantly, deal with the comments about all the bugs in the stable branch. But you could also set up comment system and make a sticky  with bugs.

The more important thing for me as a player would be, why not just have public beta testing. Unless you provide a demo version or this is a web game, there are freeloaders that will never buy your game,  but might point out bugs. Or other people that might buy your game, but shy away, because they could not play a demo of it.

Beta/Restricted: This is where I’d release new versions of my game first and have it restricted to a beta set of invited players only. The beta players can test the game version to find issues and provide feedback. It would be free.

My understanding is that the rule is mostly to avoid people flooding the search with copies of the same project. Your case seems to be that one project will be public, and one will be private.

I don’t think that will be an issue.